1. Field of the Invention
The present invention relates generally to neurons in an artificial neural network. More particularly, the present invention relates to solid state, adjustable weight synapses for controlling the interaction of the neurons in such artificial neural network.
2. Description of Related Art
There has been a great deal of interest in developing non-volatile associative electronic memories based on models of neural networks. A key feature of artificial neural networks is the vast number of synapses that must interconnect each neuron with many others.
Several different approaches have been tried for providing the vast number of synapses required in any artificial neural network. One example is an optical approach, wherein light beams can cross one another without signal interference and, hence, offer a convenient way to form multiple interconnections. However, the lenses and lasers needed for such optical interconnective devices make such systems too bulky for broad applications. (See, for example, J. Kinoshita and N. G. Palevsky, "Computing with Neural Networks," High Technology, May, 1987, 24-61, 1987).
An electrochemically regulated synapse known as the "Memistor" was developed in the 1960's by Bernard Widrow, as part of a network known as the "Adaline" network, as disclosed by B. Widrow, in the publication "Neural Network Theory, Past and Present," Paper presented at the DARPA Neural Network Study Symposium, Lincoln Labs, 1987. The Memistor is an electrochemical cell in which copper is either plated on or deplated from a carbon rod. As a result of the controlled plating and deplating of copper, the resistance of the rod is continuously adjustable from 1-10 ohms. This provides a 10:1 range of synaptic "weights." The Memistor serves well from the standpoint of trainability, surviving numerous plating and deplating cycles. However, the Memistor does not lend itself to miniaturization and the device is not practical for large-scale networks.
Metal migration is an electrochemical process related to electroplating. Metal migration takes place between conductors in an active electronic circuit in the presence of a moisture film. Under the influence of a DC voltage, metal ions dissolve from the positive conductor (the anode). The dissolved ions migrate through the moisture film (the electrolyte) and plate out on the negative conductor (the cathode). The deposit often takes the form of metallic whiskers which eventually reach the anode and create an ohmic contact.
Metal migration has been observed with all of the metals commonly used in the electronics industry, but it occurs most readily with silver (see A. Dermarderosian, "The Electrochemical Migration of Metals," Proc. 1978 Microelectronics Symp., 134-141, International Soc. for Hybrid Microelectronics, 1978). The minimum or "critical" voltage V.sub.c required to grow metallic whiskers can range from a few millivolts to over 2 volts, depending on the metal and prevailing conditions surrounding the electronic circuit. Once V.sub.c is exceeded, growth rates tend to increase linearly with (V-V.sub.c) (see P. B. Price, et al., "On the Growth Properties of Electrolytic Whiskers," ACTA Met., 6, 1968). The initial contact resistance is typically in the range of 10.sup.4 -10.sup.6 ohms, but with continued whisker growth, the contact resistance falls several orders of magnitude.
It has been observed that, depending on the medium, whisker growth between copper conductors proceeds either from the cathode to the anode (as usual), or from the anode to cathode, which is the reverse of other metals (see A. Dermarderosian, "Raw Material Evaluation through Moisture Resistance Testing," Proc. IPC, 1976). When growth proceeds from anode to cathode, the whiskers are known as "Conductive Anodic Filaments" or CAF (see J. P. Mitchell and T. L. Welsher, "Conductive anodic filament growth in printed circuit materials," Proc. Printed Circuit World Convention II, Session 2A, pp. 42-55 (1981); J. N. Lahti, R. H. Delaney, and J. N. Hines, "The characteristic wearout process in epoxy-glass printed circuits for high density electronic packaging," Proc. Reliability Phys. Symposium, San Francisco (1979); J. Lando, J. P. Mitchell, and T. L. Welsher, "Conductive anodic filaments in reinforced polymeric dielectrics; formation and prevention," ibid. (1979); T. L. Welsher, J. P. Mitchell and D. J. Lando, "CAF in composite printed circuit substrates: characterization, modeling, and a resistant material", Annual Report, Conference on Electrical Insulation and Dielectric Phenomena, 234-239 (1980)).
It has been suggested that the undesirable growth of metal whiskers observed in electronic circuits be utilized positively to provide the weighted or adjustable synapses in a neural network. Attempts to utilize metal whiskers as interconnects or neural network synapses were not successful, however, because of the inability to reverse the process of whisker growth. In order for metal whisker growth to provide practical synapse interconnects, it is crucial that the whisker growth process be reversible.
In view of the above, it would be desirable to provide electrochemically regulated synapses adapted for use in neural networks wherein the synapse is a solid state configuration which is adapted for miniaturization for use in large scale neural networks.